1. Field of the Invention
The present invention relates generally to power supplies, and more specifically to controllers for switched mode power supplies.
2. Discussion of the Related Art
Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. Conventional wall sockets provide a high voltage alternating current. In a switching power supply a high voltage alternating current (ac) input is converted to provide a well regulated direct current (dc) output through an energy transfer element. The switched mode power supply control circuit usually provides output regulation by sensing the output and controlling it in a closed loop. In operation, a switch is utilized to provide the desired output by varying the duty cycle (typically the ratio of the on time of the switch to the total switching period) of the switch in a switched mode power supply.
Requirements such as efficiency, size, weight and cost are usually taken into account when designing a switched mode power supply. Typically, the controller which controls the switching of the switched mode power supply is designed as an integrated circuit with various terminals which may function as input terminals, output terminals or both. When the switch of a switched mode power supply is integrated with the controller, two terminals of the integrated circuit correspond to the two ends of the switch. Various terminals of the integrated circuit may be utilized as the feedback terminal, a function program terminal, or an input voltage sense terminal for the controller. For some applications of the switched mode power supply, the ac input voltage is sensed to determine the zero-crossing of the ac input voltage. In general, the ac input voltage is herein also referred to as the line input voltage. The zero-crossing generally refers to when the ac input voltage crosses zero voltage. In other words, the zero-crossing refers to when the magnitude of the ac input voltage changes sign from positive to negative or from negative to positive. The zero-crossing of the line input voltage may be used for various applications. The zero-crossing of the line input voltage may be used to determine the ac line frequency or it may be used to update the internal clock of a controller of a power supply.
In one type of dimming for lighting applications, a triac dimmer circuit typically removes a portion of the ac input voltage to limit the amount of voltage and current supplied to an incandescent lamp. This is known as phase dimming because it is often convenient to designate the position of the missing voltage in terms of a fraction of the period of the ac input voltage measured in degrees. In general, the ac input voltage is a sinusoidal waveform and the period of the ac input voltage is referred to as a full line cycle. As such, half the period of the ac input voltage is referred to as a half line cycle. An entire period of the ac input voltage has 360 degrees, and a half line cycle has 180 degrees. Typically, the phase angle is a measure of how many degrees (from a reference of zero degrees) of each half line cycle the dimmer circuit removes. Although phase angle dimming works well with incandescent lamps that receive the altered ac line voltage directly, it typically creates problems for light emitting diode (LED) lamps. LED lamps require a regulated power supply to provide regulated current and voltage from the ac power line. Conventional regulated power supplies are typically designed to ignore distortions of the ac input voltage. Their purpose is to deliver a constant regulated output until a low input voltage causes them to shut off completely. As such, conventional regulated power supplies would not dim the LED lamp. Unless a power supply for an LED lamp is specially designed to recognize and respond to the voltage from a triac dimmer circuit in a desirable way, a triac dimmer is likely to produce unacceptable results such as flickering of the LED lamp, flashing of the LED lamp at high phase angles, and color shifting of the LED lamp. Thus, a power supply may include an improved conventional power supply controller that is designed to respond to a triac dimmer circuit by directly sensing the ac input voltage to determine when the ac input voltage is cut-off due to the dimming circuit. Typically, the ac input voltage is directly sensed with circuitry external to the integrated circuit of the conventional controller. The sensed ac input voltage may be received by the integrated circuit of the improved conventional controller at a terminal dedicated to receiving the sensed ac input voltage or at another terminal which performs multiple functions.
Another difficulty in using triac dimming circuits with LED lamps comes from a characteristic of the triac itself. A triac is a semiconductor component that behaves as a controlled ac switch. In other words, it behaves as an open switch to an ac voltage until it receives a trigger signal at a control terminal which causes the switch to close. The switch remains closed as long as the current through the switch is above a value referred to as the holding current. Most incandescent lamps take more than enough current from the ac power source to allow reliable and consistent operation of a triac. However, the low current taken by efficient power supplies which drive LED lamps from the ac power source may not be enough to keep a triac conducting for the expected portion of the ac line period. Therefore, conventional power supply controller designs usually rely on the power supply including a dummy load, sometimes called a bleeder circuit, to take enough extra current from the input of the power supply to keep the triac conducting after it is triggered. In general, a conventional bleeder circuit is external from the integrated circuit of the conventional power supply controller. However, use of the conventional bleeder circuit external to the conventional power supply controller requires the use of extra components with associated penalties in cost and efficiency.
Another important consideration for power supply design is the shape and phase of the input current drawn from the ac power source relative to the ac input voltage waveform. The voltage waveform of the ac power source is nominally a sinusoid. However, due to the non-linear loading that many switching power supplies present to the ac power source, the wave shape of the current drawn from the ac power source by the power supply is non-sinusoidal and/or out of phase with the ac input voltage. This leads to increased losses in the ac mains distribution system and, in many parts of the world, is now the subject of legislative or voluntary requirements that force power supply manufacturers to ensure the input current drawn by the power supply is sinusoidal and in phase with the ac input voltage waveform.
The correction of the input current waveform in this way is referred to as power factor correction (PFC) and often requires an input stage to the power supply specifically designed to perform the function of power factor correction. If the input ac current and voltage waveforms are sinusoidal and perfectly in phase, the power factor of the power supply is 1. In other words, a power factor corrected input will present a load to the ac source that is equivalent to coupling a variable resistance across the ac source. As harmonic distortion and/or phase displacement of the input current relative to the ac source voltage increase, the power factor decreases below 1. Power factor requirements typically require power factors greater than 0.9 and may have requirements for the harmonic content of the input current waveform.
Common methods to increase the power factor of a power supply include the use of a boost converter or flyback converter to establish an input current waveform close to the ideal sinusoidal shape while being in phase with the ac source voltage. Another method to increase the power factor of a power supply is to utilize a bleeder circuit. Switched mode power supplies typically include a filter capacitor which filters the high frequency current through the switch of a switched mode power supply. The bleeder circuit may facilitate the discharging of the filter capacitor which helps to pull down the voltage on the filter capacitor such that the voltage across the filter capacitor substantially follows positive magnitude of the ac input voltage. As such, the bleeder circuit helps to establish an input current waveform close to the ideal sinusoidal shape while being in phase with the ac source voltage. However, for many applications a conventional bleeder circuit is typically a circuit external to the integrated circuit of the power supply controller. Typically, the conventional bleeder circuit is implemented with a resistor which is coupled at the input of the power supply. However, as stated above, the use of a conventional bleeder circuit external to the conventional power supply controller requires the use of extra components with associated penalties in cost and efficiency.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.